Nitride-based semiconductor element and method of forming nitride-based semiconductor

ABSTRACT

A nitride-based semiconductor element having superior mass productivity and excellent element characteristics is obtained. This nitride-based semiconductor element comprises a substrate comprising a surface having projection portions, a mask layer formed to be in contact with only the projection portions of the surface of the substrate, a first nitride-based semiconductor layer formed on recess portions of the substrate and the mask layer and a nitride-based semiconductor element layer, formed on the first nitride-based semiconductor layer, having an element region. Thus, the first nitride-based semiconductor layer having low dislocation density is readily formed on the projection portions of the substrate and the mask layer through the mask layer serving for selective growth. When the nitride-based semiconductor element layer having the element region is grown on the first nitride-based semiconductor layer having low dislocation density, a nitride-based semiconductor element having excellent element characteristics can be readily obtained. The first nitride-based semiconductor layer is formed through only single growth on the substrate, whereby a nitride-based semiconductor element having excellent mass productivity is obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride-based semiconductor elementand a method of forming a nitride-based semiconductor, and morespecifically, it relates to a nitride-based semiconductor elementincluding a nitride-based semiconductor layer formed by epitaxiallateral overgrowth and a method of forming a nitride-basedsemiconductor.

2. Description of the Prior Art

In recent years, a nitride-based semiconductor element utilizing a groupIII nitride-based semiconductor is actively developed as a semiconductorelement employed for a semiconductor light-emitting device such as alight-emitting diode device or a semiconductor laser device or anelectronic device such as a transistor. In order to fabricate such anitride-based semiconductor element, a nitride-based semiconductor layeris epitaxially grown on a substrate consisting of sapphire or the like.

In this case, the substrate of sapphire or the like and thenitride-based semiconductor layer have different lattice constants andhence the nitride-based semiconductor layer grown on the substrate ofsapphire or the like has dislocations (lattice defects) verticallyextending from the substrate to the surface of the semiconductor layer.Such dislocations in the nitride-based semiconductor layer result indeterioration of the element characteristics of the semiconductorelement and reduction of the reliability thereof.

As a method of reducing the density of the aforementioned dislocationsin the nitride-based semiconductor layer, epitaxial lateral growth isgenerally proposed. This epitaxial lateral growth is disclosed inInternational Workshop on Nitride Semiconductors-IWN2000-, Nagoya,Japan, 2000, p. 79, for example.

FIGS. 29 to 33 are sectional views for illustrating a conventionalmethod of forming a nitride-based semiconductor employing epitaxiallateral overgrowth. The conventional method of forming a nitride-basedsemiconductor employing epitaxial lateral overgrowth is now describedwith reference to FIGS. 29 to 33.

First, a GaN layer 102 for serving as an underlayer is formed on asubstrate 101 consisting of sapphire or SiC, as shown in FIG. 29. Then,mask layers 103 are formed on prescribed regions of the GaN layer 102.

Then, portions of the GaN layer 102 located under regions formed with nomask layers 103 are removed by etching while etching the substrate 101by a thickness in the range not reaching the bottom surface thereofthrough the mask layers 103 serving for etching in this process. Thus,the substrate 101 is brought into a ridged shape, while stripe-patternedGaN layers 102 to be in contact substantially with the overall uppersurfaces of projection portions of the substrate 101, as shown in FIG.30.

Then, undoped GaN layers 104 are re-grown from exposed side surfaces,serving as seed crystals, of the GaN layers 102, as shown in FIG. 31.The undoped GaN layers 104 are laterally grown in an initial stage. Fromthe state shown in FIG. 31, the undoped GaN layers 104 are grown upwardwhile laterally growing on the mask layers 103 serving for selectivegrowth in this process, as shown in FIG. 32. At this time, voids 105 areformed between the undoped GaN layers 104 and the bottom surfaces ofrecess portions of the substrate 101. The undoped GaN layers 104laterally growing on the mask layers 103 coalesce into a continuousundoped GaN layer 104 having a flattened surface, as shown in FIG. 33.

In the conventional method of forming a nitride-based semiconductor, ashereinabove described, the undoped GaN layer 102 is formed by epitaxiallateral overgrowth from the exposed side surfaces of the GaN layers 102serving as seed crystals, whereby lattice defects are scarcelypropagated from the GaN layers 102 to a portion around the surface ofthe undoped GaN layer 104. Thus, the undoped GaN layer 104 reduced indislocation density is obtained. When a nitride-based semiconductorelement layer (not shown) having an element region is formed on such anundoped GaN layer 104 reduced in dislocation density, a nitride-basedsemiconductor element having excellent crystallinity can be formed.

In the aforementioned conventional method of forming a nitride-basedsemiconductor employing epitaxial lateral overgrowth, however, thesubstrate 101 is brought into the ridged shape by removing the portionsof the GaN layers 102 located under the regions formed with no masklayers 103 by etching and thereafter further etching the substrate 101.In general, therefore, the layers GaN 102, which are hardly etchednitride-based semiconductor layers, must be etched along the overallthicknesses thereof while the surface of the substrate 101 must also beetched. Thus, the etching time for bringing the substrate 101 into theridged shape is disadvantageously increased. Consequently, thenitride-based semiconductor is disadvantageously reduced in massproductivity.

In the aforementioned conventional method of forming a nitride-basedsemiconductor employing epitaxial lateral overgrowth, further, theundoped GaN layer 104 is formed by growing the GaN layers 102 serving asunderlayers on the substrate 101 and thereafter epitaxially laterallyovergrowing the GaN layers 102. Therefore, this method requires twocrystal growth steps for the GaN layers 102 and the undoped GaN layer104. In general, therefore, the nitride-based semiconductor is reducedin mass productivity also in this point.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nitride-basedsemiconductor element having superior mass productivity and excellentelement characteristics.

Another object of the present invention is to provide a method offorming a nitride-based semiconductor capable of obtaining anitride-based semiconductor layer having excellent mass productivity andlow dislocation density.

A nitride-based semiconductor element according to a first aspect of thepresent invention comprises a substrate comprising a surface havingprojection portions, a mask layer formed to be in contact with only theprojection portions of the surface of the substrate, a firstnitride-based semiconductor layer formed on recess portions of thesubstrate and the mask layer, and a nitride-based semiconductor elementlayer, formed on the first nitride-based semiconductor layer, having anelement region.

The nitride-based semiconductor element according to the first aspect isprovided with the substrate comprising a surface having projectionportions and the mask layer formed to be in contact with only theprojection portions of the surface of the substrate as hereinabovedescribed, whereby the first nitride-based semiconductor layer havinglow dislocation density can be readily formed on the recess portions ofthe substrate and the mask layer through the mask layer serving forselective growth. When the nitride-based semiconductor element layerhaving the element region is grown on the first nitride-basedsemiconductor layer having low dislocation density, a nitride-basedsemiconductor element having excellent element characteristics can bereadily obtained. Further, only the surface of the substrate may beetched for forming the projection portions. Thus, the etching time forforming the projection portions can be reduced. According to the firstaspect, further, the first nitride-based semiconductor layer can beformed through single growth on the substrate. Consequently, anitride-based semiconductor element having excellent mass productivitycan be obtained.

In the aforementioned nitride-based semiconductor element according tothe first aspect, the substrate preferably includes a substrate selectedfrom a group consisting of a sapphire substrate, a spinel substrate, anSi substrate, an SiC substrate, a GaN substrate, a GaAs substrate, a GaPsubstrate, an InP substrate, a ZrB₂ substrate and a quartz substrate. Inthis case, the substrate preferably includes a sapphire substrate, whilethe mask layer and the projection portions of the surface of thesubstrate are preferably formed in the shape of stripes being parallelto the [1-100] direction of the sapphire substrate. The substratepreferably includes an Si substrate, and the mask layer and theprojection portions of the surface of the substrate are preferablyformed in the shape of stripes being parallel to the [1-10] direction ofthe Si substrate.

The aforementioned nitride-based semiconductor element according to thefirst aspect preferably further comprises a buffer layer formed on theinterface between the recess portions of the substrate and the firstnitride-based semiconductor layer. According to this structure, thefirst nitride-based semiconductor layer having lower dislocation densitycan be formed on the buffer layer.

A nitride-based semiconductor element according to a second aspect ofthe present invention comprises an underlayer, formed on a substrate,consisting of a nitride-based semiconductor and comprising a surfacehaving projection portions, a mask layer formed to be in contact withonly the projection portions of the surface of the underlayer, a firstnitride-based semiconductor layer formed on recess portions of theunderlayer and the mask layer, and a nitride-based semiconductor elementlayer, formed on the first nitride-based semiconductor layer, having anelement region.

The nitride-based semiconductor element according to the second aspectis provided with the underlayer comprising the surface having theprojection portions and the mask layer formed to be in contact with onlythe projection portions of the surface of the underlayer as describedabove, whereby the first nitride-based semiconductor layer having lowdislocation density can be readily formed on the recess portions of theunderlayer and the mask layer through the mask layer serving forselective growth. When the nitride-based semiconductor element layerhaving the element region is grown on the first nitride-basedsemiconductor layer having low dislocation density, a nitride-basedsemiconductor element having excellent element characteristics can bereadily obtained. Only the surface of the underlayer consisting of anitride-based semiconductor may be etched for forming the projectionportions on the surface. Thus, the etching time for forming theprojection portions on the surface can be reduced, and a nitride-basedsemiconductor element having excellent mass productivity can be obtainedas a result.

The aforementioned nitride-based semiconductor element according to thesecond aspect preferably further comprises a buffer layer formed betweenthe substrate and the underlayer. According to this structure, theunderlayer consisting of a nitride-based semiconductor having lowdislocation density can be readily formed on the buffer layer.

In the aforementioned nitride-based semiconductor element according tothe second embodiment, the substrate preferably includes a substrateselected from a group consisting of a sapphire substrate, a spinelsubstrate, an Si substrate, an SiC substrate, a GaAs substrate, a GaPsubstrate, an InP substrate, a ZrB₂ substrate and a quartz substrate.

In the aforementioned nitride-based semiconductor element according tothe second aspect, the underlayer preferably includes a GaN layer, andthe mask layer and the projection portions of the surface of theunderlayer are preferably formed in the shape of stripes being parallelto the [11-20] direction or the [1-100] direction of the GaN layer.

A method of forming a nitride-based semiconductor according to a thirdaspect of the present invention comprises steps of forming projectionportions on a surface on a substrate, forming a mask layer to be incontact with only the projection portions of the surface of thesubstrate and growing a first nitride-based semiconductor layer onrecess portions of the substrate and the mask layer through the masklayer serving for selective growth.

In the method of forming a nitride-based semiconductor according to thethird embodiment, the surface having the projection portions is formedon the substrate while the mask layer is formed to be in contact withonly the projection portions of the surface of the substrate, wherebythe first nitride-based semiconductor layer having low dislocationdensity can be readily formed on the recess portions of the substrateand the mask layer when grown through the mask layer serving forselective growth. Further, only the surface of the substrate may beetched for forming the projection portions on the surface. Thus, theetching time for forming the projection portions on the surface can bereduced. According to the third aspect, further, the first nitride-basedsemiconductor layer can be formed through single growth on thesubstrate. Consequently, a method of forming a nitride-basedsemiconductor excellent in mass productivity can be obtained.

The aforementioned method of forming a nitride-based semiconductoraccording to the third aspect preferably further comprises a step offorming a buffer layer on the recess portions of the substrate inadvance of the step of growing the first nitride-based semiconductorlayer. According to this structure, the first nitride-basedsemiconductor layer having lower dislocation density can be formed onthe buffer layer.

In the aforementioned method of forming a nitride-based semiconductoraccording to the third aspect, the steps of forming the projectionportions on the surface on the substrate and forming the mask layerpreferably include a step of forming the mask layer on the surface ofthe substrate and thereafter etching the surface of the substratethrough the mask layer serving for etching in this step therebysimultaneously forming the projection portions on the surface of thesubstrate and the mask layer coming into contact with only theprojection portions of the surface. According to this method, the masklayer serving for etching simultaneously serves for selective growth,whereby the fabrication process can be simplified.

The aforementioned method of forming a nitride-based semiconductoraccording to the third aspect preferably further comprises a step ofgrowing a nitride-based semiconductor element layer having an elementregion on the first nitride-based semiconductor layer. According to thisstructure, the nitride-based semiconductor element layer having theelement region can be grown on the first nitride-based semiconductorlayer having low dislocation density, whereby a nitride-basedsemiconductor element having excellent element characteristics can bereadily formed.

In the aforementioned method of forming a nitride-based semiconductoraccording to the third aspect, the substrate preferably includes asubstrate selected from a group consisting of a sapphire substrate, aspinel substrate, an Si substrate, an SiC substrate, a GaN substrate, aGaAs substrate, a GaP substrate, an InP substrate, a ZrB₂ substrate anda quartz substrate. In this case, the substrate preferably includes asapphire substrate, and the mask layer and the projection portions ofthe surface of the substrate are preferably formed in the shape ofstripes being parallel to the [1-100] direction of the sapphiresubstrate. Alternatively, the substrate preferably includes an Sisubstrate, and the mask layer and the projection portions of the surfaceof the substrate are preferably formed in the shape of stripes beingparallel to the [1-10] direction of the Si substrate.

A method of forming a nitride-based semiconductor according to a fourthaspect of the present invention comprises steps of forming an underlayerconsisting of a nitride-based semiconductor on a substrate, formingprojection portions on a surface on the underlayer, forming a mask layerto be in contact with only the projection portions of the surface of theunderlayer and growing a first nitride-based semiconductor layer onrecess portions of the underlayer and the mask layer through the masklayer serving for selective growth.

In the method of forming a nitride-based semiconductor according to thefourth aspect, the surface having the projection portions is formed onthe underlayer consisting of a nitride-based semiconductor layerprovided on the substrate while the mask layer is formed to be incontact with only the projection portions of the surface of theunderlayer, whereby the first nitride-based semiconductor layer havinglow dislocation density can be readily formed on the recess portions ofthe underlayer and the mask layer when grown through the mask layerserving for selective growth. Further, only the surface of theunderlayer consisting of a nitride-based semiconductor may be etched forforming the projection portions on the surface. Thus, the etching timefor forming the projection portions on the surface can be reduced, and amethod of forming a nitride-based semiconductor having excellent massproductivity can be provided as a result.

The aforementioned method of forming a nitride-based semiconductoraccording to the fourth aspect preferably further comprises a step offorming a buffer layer on the substrate in advance of the step offorming the underlayer consisting of a nitride-based semiconductor.According to this structure, the underlayer consisting of anitride-based semiconductor having low dislocation density can bereadily formed on the buffer layer.

In the aforementioned method of forming a nitride-based semiconductoraccording to the fourth aspect, the steps of forming the projectionportions on the surface on the underlayer and forming the mask layerpreferably include a step of forming the mask layer on the surface ofthe underlayer and thereafter etching the surface of the underlayerthrough the mask layer serving for etching in this step therebysimultaneously forming the projection portions on the surface of theunderlayer and the mask layer coming into contact with only theprojection portions of the surface. According to this method, the masklayer serving for etching simultaneously serves for selective growth,whereby the fabrication process can be simplified.

The aforementioned method of forming a nitride-based semiconductoraccording to the fourth aspect preferably further comprises a step ofgrowing a nitride-based semiconductor element layer having an elementregion on the first nitride-based semiconductor layer. According to thisstructure, the nitride-based semiconductor element layer having theelement region can be grown on the first nitride-based semiconductorlayer having low dislocation density, whereby a nitride-basedsemiconductor element having excellent element characteristics can bereadily formed.

In the aforementioned method of forming a nitride-based semiconductoraccording to the fourth aspect, the substrate preferably includes asubstrate selected from a group consisting of a sapphire substrate, aspinel substrate, an Si substrate, an SiC substrate, a GaAs substrate, aGaP substrate, an InP substrate, a ZrB₂ substrate and a quartzsubstrate.

In the aforementioned method of forming a nitride-based semiconductoraccording to the fourth aspect, the underlayer preferably includes a GaNlayer, and the mask layer and the projection portions of the surface ofthe underlayer are preferably formed in the shape of stripes beingparallel to the [11-20] direction or the [1-100] direction of the GaNlayer.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 6 are sectional views for illustrating a method of forming anitride-based semiconductor according to a first embodiment of thepresent invention;

FIG. 7 is a perspective view showing a semiconductor laser devicefabricated with the method of forming a nitride-based semiconductoraccording to the first embodiment of the present invention;

FIGS. 8 to 13 are sectional views for illustrating a method of forming anitride-based semiconductor according to a second embodiment of thepresent invention;

FIG. 14 is a perspective view showing a semiconductor laser devicefabricated with the method of forming a nitride-based semiconductoraccording to the second embodiment of the present invention;

FIGS. 15 to 20 are sectional views for illustrating a method of forminga nitride-based semiconductor according to a third embodiment of thepresent invention;

FIG. 21 is a perspective view showing a semiconductor laser devicefabricated with the method of forming a nitride-based semiconductoraccording to the third embodiment of the present invention;

FIGS. 22 to 27 are sectional views for illustrating a method of forminga nitride-based semiconductor according to a fourth embodiment of thepresent invention;

FIG. 28 is a perspective view showing a semiconductor laser devicefabricated with the method of forming a nitride-based semiconductoraccording to the fourth embodiment of the present invention; and

FIGS. 29 to 33 are sectional views for illustrating a conventionalmethod of forming a nitride-based semiconductor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are now described with reference tothe drawings.

First Embodiment

A method of forming a nitride-based semiconductor according to a firstembodiment of the present invention is described with reference to FIGS.1 to 6.

First, striped mask layers 2 of SiO₂ having a thickness of about 0.5 μmare formed on a sapphire (0001) plane substrate 1 (hereinafter referredto as “sapphire substrate 1”), as shown in FIG. 1. The stripe patternsof the mask layers 2 are formed in a cycle of about 7 μm so that thewidth of the mask layers 2 is about 5 μm and the interval between eachadjacent pair of mask layers 2 (the width of mask openings) is about 2μm. The striped mask layers 2 are formed in parallel to the [1-100]direction of the sapphire substrate 1. The sapphire substrate 1 is anexample of the “substrate” according to the present invention.

The mask layers 2 are employed as masks for etching the surface of thesapphire substrate 1 by a thickness of about 1 μm through RIE (reactiveion etching) or the like. Thus, the surface of the sapphire substrate 1is brought into an uneven shape, as shown in FIG. 2. The shape of theprojection portions varies with etching conditions, such that upperparts of recess portions may be larger or smaller in width than bottomparts thereof. In the following description, side surfaces of the recessportions of the etched sapphire substrate 1 are nearly perpendicular tothe upper surface of projection portions of the sapphire substrate 1.The uneven surface of the sapphire substrate 1 has ridges of a height ofabout 1 μm and grooves of a terrace width of about 2 μm, and they areformed in parallel to the [1-100] direction of the sapphire substrate 1.

Then, low-temperature buffer layers 3 of GaN having a thickness of about15 nm are grown by a crystal growth method such as MOVPE (metal organicvapor phase epitaxy) to be in contact substantially with the overallbottom surfaces of the recess portions of the sapphire substrate 1, asshown in FIG. 3. In this case, the low-temperature buffer layers 3 arehardly formed on the mask layers 2 consisting of SiO₂. Alternatively,the low-temperature buffer layers 3 may be formed not only on the bottomsurfaces but also on the side surfaces of the recess portions. Thelow-temperature buffer layers 3 may not be formed on the overall bottomsurfaces of the recess portions but may be partially formed on thebottom surfaces of the recess portions. The low-temperature bufferlayers 3 are examples of the “buffer layer” according to the presentinvention.

Then, undoped GaN layers 4 are grown on the low-temperature bufferlayers 3 consisting of GaN. In this case, the low-temperature bufferlayers 3 and the undoped GaN layers 4 are successively grown withouttaking the sapphire substrate 1 out of the growth apparatus. In aninitial stage, the undoped GaN layers 4 are vertically (upwardly) grownon the low-temperature buffer layers 3. If the low-temperature bufferlayers 3 are formed on the side surfaces of the recess portions, theundoped GaN layers 4 are grown laterally from the low-temperature bufferlayers 3 formed on the side surfaces. When this growth is furthercontinued, undoped GaN layers 4 having facets on side surfaces thereofare formed on the recess portions, as shown in FIG. 4. The undoped GaNlayers 4 are examples of the “first nitride-based semiconductor layer”according to the present invention.

From the state shown in FIG. 4, the undoped GaN layers 4 are laterallygrown on the mask layers 2, as shown in FIG. 5. The undoped GaN layers 4laterally grown on the mask layers 2 coalesce into a continuous undopedGaN layer 4 of about 5 μm in thickness having a flattened surface, asshown in FIG. 6.

In the method of forming a nitride-based semiconductor according to thefirst embodiment, the undoped GaN layers 4 are grown from the recessportions of the sapphire substrate 1 as hereinabove described, wherebydislocations of the undoped GaN layers 4 are bent in the in-planedirection of the (0001) plane of the undoped GaN layers 4 when theundoped GaN layers 4 are laterally grown from the low-temperature bufferlayers 3 formed on the side surfaces of the recess portions or on themask layers 2. Thus, the dislocation density can be reduced around thesurfaces of the undoped GaN layers 4.

In the method of forming a nitride-based semiconductor according to thefirst embodiment, the surface of the sapphire substrate 1 is broughtinto an uneven shape as hereinabove described, whereby only the surfaceof the sapphire substrate 1 may be etched through the mask layers 2serving for etching. Thus, the etching time for forming the projectionportions on the surface on the sapphire substrate 1 can be reduced ascompared with the conventional process of forming the projectionportions on the surface shown in FIG. 30. Consequently, a method offorming a nitride-based semiconductor having excellent mass productivitycan be obtained.

In the method of forming a nitride-based semiconductor according to thefirst embodiment, growth of the low-temperature buffer layers 3 of GaNformed on the recess portions of the sapphire substrate 1 and selectivegrowth of the undoped GaN layer 4 are successively performed withouttaking the sapphire substrate 1 out of the growth apparatus, ashereinabove described. Thus, the undoped GaN layer 4 having lowdislocation density can be formed through a single growth step. A methodof forming a nitride-based semiconductor having excellent massproductivity can be obtained also in this point.

In the method of forming a nitride-based semiconductor according to thefirst embodiment, further, the undoped GaN layer 4 is grown on thelow-temperature buffer layers 3 provided on the sapphire substrate 1,whereby the undoped GaN layer 4 can be grown in lower dislocationdensity as compared with that directly grown on the sapphire substrate1.

FIG. 7 is a perspective view showing a semiconductor laser devicefabricated with the aforementioned method of forming a nitride-basedsemiconductor according to the first embodiment. The structure of thesemiconductor laser device fabricated with the method of forming anitride-based semiconductor according to the first embodiment is nowdescribed with reference to FIG. 7.

In the structure of the semiconductor laser device according to thefirst embodiment, an n-type contact layer 5 of n-type GaN having athickness of about 4 μm is formed on the undoped GaN layer 4 accordingto the first embodiment shown in FIG. 6, as shown in FIG. 7. Ananti-cracking layer 6 of n-type AlGaInN having a thickness of about 0.1μm, an n-type second cladding layer 7 of n-type AlGaN having a thicknessof about 0.45 μm, an n-type first cladding layer 8 of n-type GaN havinga thickness of about 50 nm (about 0.05 μm) and a multiple quantum well(MQW) emission layer 9 of GaInN are successively formed on the n-typecontact layer 5. The MQW emission layer 9 is formed by alternatelystacking five undoped GaN barrier layers of about 4 nm in thickness andfour compressive strain undoped GaInN well layers of about 4 nm inthickness.

A p-type first cladding layer 10 of p-type GaN having a thickness ofabout 40 nm (about 0.04 μm) is formed on the MQW emission layer 9. Amesa (trapezoidal) p-type second cladding layer 11 of p-type AlGaNhaving a height of about 0.45 μm is formed on the p-type first claddinglayer 10. Current blocking layers 12 of n-type GaN having a thickness ofabout 0.2 μm are formed to cover regions on the p-type first claddinglayer 10 other than that formed with the p-type second cladding layer 11and the side surfaces of the mesa p-type second cladding layer 11 whileexposing the upper surface of the p-type second cladding layer 11. Ap-type contact layer 13 of p-type GaN having a thickness of about 3 μmto about 5 μm is formed on the current blocking layers 12 to be incontact with the exposed upper surface of the p-type second claddinglayer 11.

The layers from the p-type contact layer 13 to the n-type contact layer5 are partially removed. Protective films 14 of an insulator such asSiO₂ or SiN are formed to cover parts of the exposed surface of then-type contact layer 5 and the exposed side surfaces of theanti-cracking layer 6, the n-type second cladding layer 7, the n-typefirst cladding layer 8, the MQW emission layer 9, the p-type firstcladding layer 10, the current blocking layers 12 and the p-type contactlayer 13.

A p-side electrode 15 is formed on the upper surface of the p-typecontact layer 13 while an n-side electrode 16 is formed on the partiallyremoved and exposed surface of the n-type contact layer 5.

The n-type contact layer 5, the anti-cracking layer 6, the n-type secondcladding layer 7, the n-type first cladding layer 8, the MQW emissionlayer 9, the p-type first cladding layer 10, the p-type second claddinglayer 11, the current blocking layers 12 and the n-type contact layer 13are examples of the “nitride-based semiconductor element layer having anelement region” according to the present invention.

In the semiconductor laser device according to the first embodiment, theundoped GaN layer 4 having excellent mass productivity and lowdislocation density formed by the method of forming a nitride-basedsemiconductor according to the first embodiment shown in FIG. 1 to 6 isemployed as an underlayer for forming the layers 5 to 13 thereon ashereinabove described, whereby excellent crystallinity can beimplemented in the layers 5 to 13. Consequently, a semiconductor laserdevice having excellent mass productivity and excellent devicecharacteristics can be obtained according to the first embodiment.

Second Embodiment

Referring to FIGS. 8 to 13, an n-type Si (111) plane substrate 21(hereinafter referred to as “Si substrate 21”) having conductivity isemployed in a second embodiment of the present invention in place of theinsulating sapphire substrate 1 in the first embodiment. A method offorming a nitride-based semiconductor according to the second embodimentis now described with reference to FIGS. 8 to 13.

According to the second embodiment, striped mask layers 22 of SiO₂having a thickness of about 0.5 μm are formed on the n-type Si substrate21, as shown in FIG. 8. The stripe patterns of the mask layers 22 areformed in a cycle of about 7 μm, so that the mask layers 22 are about 5μm in width and the interval between each adjacent pair of mask layers22 (the width of mask openings) is about 2 μm. The striped mask layers22 are formed in parallel to the [1-10] direction of the Si substrate21. The Si substrate 21 is an example of the “substrate” according tothe present invention.

The mask layers 22 are employed as masks for etching the surface of theSi substrate 21 by a thickness of about 1 μm through wet etching or thelike. Thus, the surface of the Si substrate 21 is brought into an unevenshape, as shown in FIG. 9. The shape of the projection portions varieswith etching conditions, such that upper parts of recess portions may belarger or smaller in width than bottom parts thereof. In the followingdescription, projection portions of the etched Si substrate 21 are inthe form of a mesa (trapezoid). The uneven surface of the Si substrate21 has ridges of a height of about 1 μm, and they are formed in parallelto the [1-10] direction of the Si substrate 21.

Then, buffer layers 23 of Si-doped AlGaN having a thickness of about 15nm are grown by a crystal growth method such as MOVPE to be in contactsubstantially with the overall bottom surfaces of the recess portions ofthe Si substrate 21, as shown in FIG. 10. In this case, the bufferlayers 23 are hardly formed on the mask layers 22 consisting of SiO₂.Alternatively, the buffer layers 23 may be formed not only on the bottomsurfaces but also on the side surfaces of the recess portions. Thebuffer layers 23 may not be formed on the overall bottom surfaces of therecess portions but may be partially formed on the bottom surfaces ofthe recess portions.

Then, Si-doped GaN layers 24 are grown on the buffer layers 23consisting of Si-doped AlGaN. In this case, the buffer layers 23 and theSi-doped GaN layers 24 are successively grown without taking the Sisubstrate 21 out of the growth apparatus. In an initial stage, theSi-doped GaN layers 24 are vertically (upwardly) grown on the bufferlayers 23. If the buffer layers 23 are formed on the side surfaces ofthe recess portions, the Si-doped GaN layers 24 are grown laterally fromthe buffer layers 23 formed on the side surfaces. When this growth isfurther continued, Si-doped GaN layers 24 having facets on side surfacesthereof are formed on the recess portions, as shown in FIG. 11. TheSi-doped GaN layers 24 are examples of the “first nitride-basedsemiconductor layer” according to the present invention.

From the state shown in FIG. 11, the Si-doped GaN layers 24 arelaterally grown on the mask layers 22, as shown in FIG. 12. The Si-dopedGaN layers 24 laterally grown on the mask layers 22 coalesce into acontinuous Si-doped GaN layer 24 of about 5 μm in thickness having aflattened surface, as shown in FIG. 13.

In the method of forming a nitride-based semiconductor according to thesecond embodiment, the Si-doped GaN layers 24 are grown from the recessportions of the Si substrate 21, whereby dislocations of the Si-dopedGaN layers 24 are bent in the in-plane direction of the (0001) plane ofthe Si-doped GaN layers 24 when the Si-doped GaN layers 24 are laterallygrown from the buffer layers 23 formed on the side surfaces of therecess portions or on the mask layers 22. Thus, the dislocation densitycan be reduced around the surfaces of the Si-doped GaN layers 24.

In the method of forming a nitride-based semiconductor according to thesecond embodiment, the surface of the Si substrate 21 is brought into anuneven shape similarly to the first embodiment, whereby only the surfaceof the Si substrate 21 may be etched through the mask layers 22 servingfor etching. Thus, the etching time for forming the projection portionson the surface on the Si substrate 21 can be reduced as compared withthe conventional process of forming the projection portions on thesurface shown in FIG. 30. Consequently, a method of forming anitride-based semiconductor having excellent mass productivity can beobtained.

In the method of forming a nitride-based semiconductor according to thesecond embodiment, growth of the buffer layers 23 of Si-doped AlGaNformed on the Si substrate 21 and selective growth of the Si-doped GaNlayer 24 are successively performed without taking the Si substrate 21out of the growth apparatus, similarly to the first embodiment. Thus,the Si-doped GaN layer 24 having low dislocation density can be formedthrough a single growth step. A method of forming a nitride-basedsemiconductor having excellent mass productivity can be obtained also inthis point.

In the method of forming a nitride-based semiconductor according to thesecond embodiment, further, the Si-doped GaN layer 24 is grown on thebuffer layers 23 provided on the Si substrate 21, whereby the Si-dopedGaN layer 24 can be grown in lower dislocation density as compared withthat directly grown on the Si substrate 21.

FIG. 14 is a perspective view showing a semiconductor laser devicefabricated with the aforementioned method of forming a nitride-basedsemiconductor according to the second embodiment. The structure of thesemiconductor laser device fabricated with the method of forming anitride-based semiconductor according to the second embodiment is nowdescribed with reference to FIG. 14.

In the structure of the semiconductor laser device according to thesecond embodiment, an anti-cracking layer 25 of n-type AlGaInN having athickness of about 0.1 μm, an n-type second cladding layer 26 of n-typeAlGaN having a thickness of about 0.45 μm, an n-type first claddinglayer 27 of n-type GaN having a thickness of about 50 nm (about 0.05 μm)and a multiple quantum well (MQW) emission layer 28 of GaInN aresuccessively formed on the Si-doped GaN layer 24 according to the secondembodiment shown in FIG. 13, as shown in FIG. 14. The MQW emission layer28 is formed by alternately stacking five undoped GaN barrier layers ofabout 4 nm in thickness and four compressive strain undoped GaInN welllayers of about 4 nm in thickness.

A p-type first cladding layer 29 of p-type GaN having a thickness ofabout 40 nm (about 0.04 μm) is formed on the MQW emission layer 28. Amesa (trapezoidal) p-type second cladding layer 30 of p-type AlGaNhaving a height of about 0.45 μm is formed on the p-type first claddinglayer 29. Current blocking layers 31 of n-type GaN having a thickness ofabout 0.2 μm are formed to cover regions on the p-type first claddinglayer 29 other than that formed with the p-type second cladding layer 30and the side surfaces of the mesa p-type second cladding layer 30 whileexposing the upper surface of the p-type second cladding layer 30. Ap-type contact layer 32 of p-type GaN having a thickness of about 3 μmto about 5 μm is formed on the current blocking layers 31 to be incontact with the exposed upper surface of the p-type second claddinglayer 30.

A p-side electrode 33 is formed on a projection portion of the p-typecontact layer 32 reflecting the mesa shape of the p-type second claddinglayer 30. According to the second embodiment, the Si substrate 21 hasconductivity dissimilarly to the sapphire substrate 1 according to thefirst embodiment, and hence an n-side electrode 34 is formed on the backsurface of the Si substrate 21.

The anti-cracking layer 25, the n-type second cladding layer 26, then-type first cladding layer 27, the MQW emission layer 28, the p-typefirst cladding layer 29, the p-type second cladding layer 30, thecurrent blocking layers 31 and the n-type contact layer 32 are examplesof the “nitride-based semiconductor element layer having an elementregion” according to the present invention.

In the semiconductor laser device according to the second embodiment,the Si-doped GaN layer 24 having excellent mass productivity and lowdislocation density formed by the method of forming a nitride-basedsemiconductor according to the second embodiment shown in FIG. 8 to 13is employed as an underlayer for forming the layers 25 to 32 thereon ashereinabove described, whereby excellent crystallinity can beimplemented in the layers 25 to 32. Consequently, a semiconductor laserdevice having excellent mass productivity and excellent devicecharacteristics can be obtained.

While the sapphire substrate 1 and the Si substrate 21 are employed inthe aforementioned first and second embodiments respectively, forexample, the present invention is not restricted to this but a spinelsubstrate, an SiC substrate, a GaAs substrate, a GaP substrate, an InPsubstrate, a ZrB₂ substrate or a quartz substrate may alternatively beemployed.

Further alternatively, a GaN substrate may be employed in each of theaforementioned first and second embodiments. In this case, thelow-temperature buffer layers 3 or the buffer layers 23 may notnecessarily be formed.

Third Embodiment

Referring to FIGS. 15 to 20, epitaxial lateral overgrowth is performedthrough an underlayer 43 comprising a surface having projection portionsformed on a sapphire (0001) plane substrate 41 (hereinafter referred toas “sapphire substrate 41”) in a third embodiment of the presentinvention. A method of forming a nitride semiconductor according to thethird embodiment is now described in detail with reference to FIGS. 15to 20.

First, low-temperature buffer layers 42 of AlGaN having a thickness ofabout 15 nm and the underlayer 43 of undoped GaN having a thickness ofabout 2 μm are formed on the sapphire substrate 41 by a crystal growthmethod such as MOVPE, as shown in FIG. 15. The sapphire substrate 41 isan example of the “substrate” according to the present invention. Thelow-temperature buffer layers 42 are examples of the “buffer layer”according to the present invention.

Striped mask layers 44 of SiO₂ having a thickness of about 0.5 μm areformed on the underlayer 43. The stripe patterns of the mask layers 44are formed in a cycle of about 6 μm so that the width of the mask layers44 is about 5 μm and the interval between each adjacent pair of masklayers 44 (the width of mask openings) is about 1 μm. The striped masklayers 44 are formed in parallel to the [11-20] direction of theunderlayer 43 consisting of GaN.

The mask layers 44 are employed as masks for etching the surface of theunderlayer 43 by a thickness of about 1 μm through RIE or the like.Thus, the surface of the underlayer 43 is brought into an uneven shape,as shown in FIG. 16. The shape of the projection portions varies withetching conditions, such that upper parts of recess portions may belarger or smaller in width than bottom parts thereof. In the followingdescription, projection portions of the etched underlayer 43 are in theform of a mesa (trapezoid). The uneven surface of the underlayer 43 hasridges of a height of about 1 μm, and they are formed in parallel to the[11-20] direction of the underlayer 43 consisting of undoped GaN.

Then, undoped GaN layers 45 are re-grown from the bottom and sidesurfaces, serving as seed crystals, of the exposed recess portions ofthe underlayer 43 consisting of undoped GaN, as shown in FIG. 17. In aninitial stage, the undoped GaN layers 45 are vertically (upwardly) grownfrom the bottom surfaces of the recess portions of the underlayer 43 andalso laterally grown from the side surfaces of the recess portions ofthe underlayer 43, as shown in FIGS. 17 and 18. The undoped GaN layers45 are examples of the “first nitride-based semiconductor layer”according to the present invention.

From the state shown in FIG. 18, the undoped GaN layers 45 are laterallygrown on the mask layers 44, as shown in FIG. 19. The undoped GaN layers45 laterally grown on the mask layers 44 coalesce into a continuousundoped GaN layer 45 of about 5 μm in thickness having a flattenedsurface, as shown in FIG. 20.

In the method of forming a nitride-based semiconductor according to thethird embodiment, the undoped GaN layers 45 are grown from the bottomand side surfaces, serving as seed crystals, of the recess portions ofthe underlayer 43 consisting of undoped GaN as hereinabove described,whereby dislocations of the undoped GaN layers 45 are bent in thein-plane direction of the (0001) plane of the undoped GaN layers 45 whenthe undoped GaN layers 45 are laterally grown from the side surfaces ofthe recess portions of the underlayer 43 or on the mask layers 44. Thus,the dislocation density can be reduced around the surfaces of theundoped GaN layers 45.

In the method of forming a nitride-based semiconductor according to thethird embodiment, the surface of the underlayer 43 is brought into anuneven shape as hereinabove described, whereby only the surface of theunderlayer 43 may be etched. Thus, the etching time for forming theprojection portions on the surface on the underlayer 43 can be reducedas compared with the conventional process of forming the projectionportions on the surface shown in FIG. 30. Consequently, a method offorming a nitride-based semiconductor having excellent mass productivitycan be obtained similarly to the first and second embodiments.

In the method of forming a nitride-based semiconductor according to thethird embodiment, the underlayer 43 consisting of undoped GaN is grownafter forming the low-temperature buffer layers 42 on the sapphiresubstrate 41, whereby the underlayer 43 having low dislocation densitycan be readily formed.

FIG. 21 is a perspective view showing a semiconductor laser devicefabricated with the aforementioned method of forming a nitride-basedsemiconductor according to the third embodiment. The structure of thesemiconductor laser device fabricated with the method of forming anitride-based semiconductor according to the third embodiment is nowdescribed with reference to FIG. 21.

In the structure of the semiconductor laser device according to thethird embodiment, an n-type contact layer 5, an anti-cracking layer 6,an n-type second cladding layer 7, an n-type first cladding layer 8, anMQW emission layer 9, a p-type first cladding layer 10, a p-type secondcladding layer 11, current blocking layers 12, a p-type contact layer 13and protective films 14 are formed on the undoped layer GaN layer 45shown in FIG. 20, similarly to the first embodiment. The compositionsand thicknesses of the layers 5 to 13 and the protective films 14 aresimilar to those in the first embodiment.

A p-side electrode 15 is formed on the upper surface of the p-typecontact layer 13 while an n-side electrode 16 is formed on a partiallyremoved and exposed surface of the n-type contact layer 5.

In the semiconductor laser device according to the third embodiment, theundoped GaN layer 45 having excellent mass productivity and lowdislocation density formed by the method of forming a nitride-basedsemiconductor according to the third embodiment shown in FIG. 15 to 20is employed as an underlayer for forming the layers 5 to 13 thereon ashereinabove described, whereby excellent crystallinity can beimplemented in the layers 5 to 13. Consequently, a semiconductor laserdevice having excellent mass productivity and excellent devicecharacteristics can be obtained.

Fourth Embodiment

Referring to FIGS. 22 to 27, an n-type SiC (0001) plane substrate 51(hereinafter referred to as “SiC substrate 51”) having conductivity isemployed in a fourth embodiment of the present invention in place of theinsulating sapphire substrate 41 employed in the third embodiment. Amethod of forming a nitride semiconductor according to the fourthembodiment is now described in detail with reference to FIGS. 22 to 27.

First, buffer layers 52 of Si-doped AlGaN having a thickness of about 15nm and an underlayer 53 of Si-doped GaN having a thickness of about 2 μmare formed on the n-type SiC substrate 51 by a crystal growth methodsuch as MOVPE, as shown in FIG. 22. The SiC substrate 51 is an exampleof the “substrate” according to the present invention.

Striped mask layers 54 of SiO₂ having a thickness of about 0.5 μm areformed on the underlayer 53. The stripe patterns of the mask layers 54are formed in a cycle of about 6 μm so that the width of the mask layers54 is about 5 μm and the interval between each adjacent pair of masklayers 54 (the width of mask openings) is about 1 μm. The striped masklayers 54 are formed in parallel to the [11-20] direction of theunderlayer 53 consisting of Si-doped GaN.

The mask layers 54 are employed as masks for etching the surface of theunderlayer 53 by a thickness of about 1 μm through RIE or the like.Thus, the surface of the underlayer 53 is brought into an uneven shape,as shown in FIG. 23. The shape of the projection portions varies withetching conditions, such that upper parts of recess portions may belarger or smaller in width than bottom parts thereof. In the followingdescription, projection portions of the etched underlayer 53 are in theform of a mesa (trapezoid). The uneven surface of the underlayer 53 hasridges of a height of about 1 μm, and they are formed in parallel to the[11-20] direction of the underlayer 53 consisting of Si-doped GaN.

Then, Si-doped GaN layers 55 are re-grown from the bottom and sidesurfaces, serving as seed crystals, of the exposed recess portions ofthe underlayer 53 consisting of Si-doped GaN, as shown in FIG. 24. In aninitial stage, the Si-doped GaN layers 55 are vertically (upwardly)grown from the bottom surfaces of the recess portions of the underlayer53 and also laterally grown from the side surfaces of the recessportions of the underlayer 53, as shown in FIGS. 24 and 25. The Si-dopedGaN layers 55 are examples of the “first nitride-based semiconductorlayer” according to the present invention.

From the state shown in FIG. 25, the Si-doped GaN layers 55 arelaterally grown on the mask layers 54, as shown in FIG. 26. The Si-dopedGaN layers 55 laterally grown on the mask layers 54 coalesce into acontinuous Si-doped GaN layer 55 of about 5 μm in thickness having aflattened surface, as shown in FIG. 27.

In the method of forming a nitride-based semiconductor according to thefourth embodiment, the Si-doped GaN layers 55 are grown from the bottomand side surfaces, serving as seed crystals, of the recess portions ofthe underlayer 43 consisting of Si-doped GaN as hereinabove described,whereby dislocations of the Si-doped GaN layers 55 are bent in thein-plane direction of the (0001) plane of the Si-doped GaN layers 55when the Si-doped GaN layers 55 are laterally grown from the sidesurfaces of the recess portions of the underlayer 53 or on the masklayers 54. Thus, the dislocation density can be reduced around thesurfaces of the Si-doped GaN layers 55.

In the method of forming a nitride-based semiconductor according to thefourth embodiment, the surface of the underlayer 53 is brought into anuneven shape as hereinabove described, whereby only the surface of theunderlayer 53 may be etched. Thus, the etching time for forming theprojection portions on the surface on the underlayer 53 can be reducedas compared with the conventional process of forming the projectionportions on the surface shown in FIG. 30. Consequently, a method offorming a nitride-based semiconductor having excellent mass productivitycan be obtained similarly to the first to third embodiments.

In the method of forming a nitride-based semiconductor according to thefourth embodiment, the underlayer 53 consisting of Si-doped GaN is grownafter forming the buffer layers 52 on the SiC substrate 51, whereby theunderlayer 53 having low dislocation density can be readily formed.

FIG. 28 is a perspective view showing a semiconductor laser devicefabricated with the aforementioned method of forming a nitride-basedsemiconductor according to the fourth embodiment. The structure of thesemiconductor laser device fabricated with the method of forming anitride-based semiconductor according to the fourth embodiment is nowdescribed with reference to FIG. 28.

In the structure of the semiconductor laser device according to thefourth embodiment, an anti-cracking layer 25, an n-type second claddinglayer 26, an n-type first cladding layer 27, an MQW emission layer 28, ap-type first cladding layer 29, a p-type second cladding layer 30,current blocking layers 31 and a p-type contact layer 32 are formed onthe Si-doped layer GaN layer 55 shown in FIG. 27, similarly to thesecond embodiment. The compositions and thicknesses of the layers 25 to32 are similar to those in the second embodiment.

A p-side electrode 33 is formed on a projection portion of the p-typecontact layer 32 reflecting the mesa shape of the p-type second claddinglayer 30. The SiC substrate 51 has conductivity, and hence an n-sideelectrode 34 is formed on the back surface of the SiC substrate 51.

In the semiconductor laser device according to the fourth embodiment,the Si-doped GaN layer 55 having excellent mass productivity and lowdislocation density formed by the method of forming a nitride-basedsemiconductor according to the fourth embodiment shown in FIG. 22 to 27is employed as an underlayer for forming the layers 25 to 32 thereon ashereinabove described, whereby excellent crystallinity can beimplemented in the layers 25 to 32. Consequently, a semiconductor laserdevice having excellent mass productivity and excellent devicecharacteristics can be obtained.

While the sapphire substrate 41 and the SiC substrate 51 are employed inthe aforementioned third and fourth embodiments respectively, forexample, the present invention is not restricted to this but a spinelsubstrate, a GaN substrate, a GaAs substrate, a GaP substrate, an InPsubstrate, a ZrB₂ substrate or a quartz substrate may alternatively beemployed.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

While the recess portions of the surfaces of the sapphire substrate 1,the Si substrate 21 and the underlayers 43 and 53 are formed in theheight of about 1 μm in the aforementioned first to fourth embodiments,the present invention is not restricted to this but the height of therecess portions is preferably set in the range of several nm to severalμm not reaching the bottom surfaces of the sapphire substrate 1, the Sisubstrate 21 and the underlayers 43 and 53.

While the striped mask layers 2, 22, 44 and 54 are formed in parallelwith the [1-100] direction of the sapphire substrate 1, the [1-10]direction of the Si substrate 21 and the [11-20] directions of the GaNunderlayers 43 and 53 in the aforementioned first to fourth embodimentsrespectively, the present invention is not restricted to this but thestriped mask layers may alternatively be formed in a direction differentfrom the aforementioned ones. For example, the mask layers 44 and 45according to the third and fourth embodiments may be formed in parallelwith the [1-100] directions of the GaN underlayers 43 and 53.

While the projection portions of the surfaces of the sapphire substrate1, the Si substrate 21 and the underlayers 43 and 53 are formed inparallel with the [1-100] direction of the sapphire substrate 1, the[1-10] direction of the Si substrate 21 and the [11-20] directions ofthe GaN underlayers 43 and 53 in the first to fourth embodimentsrespectively, the present invention is not restricted to this but theprojection portions of the surface may be formed in a directiondifferent from the above. For example, the projection portions of thesurfaces of the underlayers 43 and 53 according to the third and fourthembodiments may alternatively be formed in parallel with the [1-100]directions of the GaN underlayers 43 and 53.

While the mask layers 2, 22, 44 and 54 and the openings thereof areformed in a striped shape in the aforementioned first to fourthembodiments, the present invention is not restricted to this but themask layers may alternatively be formed in a circular, hexagonal ortriangular shape, and the openings thereof may also be formed in acircular, hexagonal or triangular shape. When the mask layers and theopenings thereof are formed in a hexagonal or triangular shape, thesides of the hexagons or triangles may match with any crystalorientation.

While the recess and projection portions of the surfaces of the sapphiresubstrate 1, the Si substrate 21 and the underlayers 43 and 53 areformed in a striped shape in the aforementioned first to fourthembodiments, the present invention is not restricted to this but therecess portions or the projection portions of the surfaces of thesapphire substrate 1, the Si substrate 21 and the underlayers 43 and 53may alternatively be formed in a circular, hexagonal or triangularshape. When the recess or projection portions are formed in a hexagonalor triangular shape, the sides of the hexagons or triangles may matchwith any crystal orientation.

While the nitride-based semiconductors are employed for preparingsemiconductor laser devices in the aforementioned first to fourthembodiments, the present invention is not restricted to this but alsoapplicable to another device such as a light emitting diode device or atransistor employing a nitride-based semiconductor.

In each of the aforementioned first to fourth embodiments, thenitride-based semiconductor may have a wurtzite crystal structure or azinc blende crystal structure.

While crystal growth of each nitride-based semiconductor layer isperformed by MOVPE in the aforementioned first to fourth embodiments,the present invention is not restricted to this but crystal growth mayalternatively be performed by HVPE or gas source MBE employing TMAl,TMGa, TMIn, NH₃, SiH or Cp₂Mg as source gas.

In each of the first to fourth embodiments, the bottom surfaces of therecess portions of the surface of the sapphire substrate 1, the Sisubstrate 21 or the underlayer 43 or 53 are preferably formed in a widthwithin the range of several 100 nm to several 10 μm.

While the n-type first cladding layers 8 or 27 and the p-type firstcladding layers 10 or 29 consist of GaN in the aforementioned first tofourth embodiments, the present invention is not restricted to this butthe first cladding layers may consist of other materials having a widerbandgap than the MQW emission layer. For example, AlGaN such asAl_(0.01)Ga_(0.99)N, InGaN such as In_(0.01)Ga_(0.99)N, or AlGaInN suchas Al_(0.01)Ga_(0.98)In_(0.01)N may be employed as the materialsconstituting the first cladding layers.

1-22. (canceled)
 23. A nitride-based semiconductor light-emitting devicecomprising: a substrate comprising a surface having projection portionsand recess portions; a buffer layer formed on bottom surfaces of saidrecess portions; and a nitride-based semiconductor layer formed on saidbuffer layer.
 24. The nitride-based semiconductor light-emitting deviceaccording to claim 23, wherein said nitride-based semiconductor layerhas a flat surface.
 25. The nitride-based semiconductor light-emittingdevice according to claim 23, wherein said substrate includes asubstrate selected from a group consisting of a sapphire substrate, anSiC substrate, a GaN substrate, a GaAs substrate, a GaP substrate, anInP substrate, a ZrB₂ substrate and a quartz substrate.
 26. Thenitride-based semiconductor light-emitting device according to claim 23,further comprising an n-type layer, an emission layer, and a p-typelayer, formed on said nitride-based semiconductor layer.
 27. Thenitride-based semiconductor light-emitting device according to claim 23,wherein said recess portions or said projection portions are formed in acircular, hexagonal or triangular shape in a plan view.
 28. Thenitride-based semiconductor light-emitting device according to claim 23,wherein said bottom surfaces of said recess portions are formed in awidth within the range of several 100 nm to several 10 μm and saidrecess portions are formed in a height within the range of several nm toseveral μm.
 29. A nitride-based semiconductor light-emitting devicecomprising: a substrate comprising a surface having projection portionsand recess portions; a buffer layer formed on bottom surfaces of saidrecess portions; and a nitride-based semiconductor layer formed on saidbuffer layer, wherein the width of said recess portions is larger thanthe height of said recess portions.